Evolution__3rd_Edition

608 PART 5 / Macroevolution
Figure 21.10
The modern horseshoe
“crab” (in fact a chelicerate,
not a crustacean) Limulus
polyphemus, which lives
along the east coast of the
USA, is a living fossil. It is
morphologically very similar
to forms that lived about
200 million years ago, and
not all that different from
Cambrian species. Redrawn,
by permission of the publisher,
from Newell (1959).
Character scores are arbitrary
Cambrian
Aglaspis
eatoni
Silurian
Pseudoniscus
roosewelti
Devonian
Belinurus
alleganyensis
Carboniferous
Prestwichinella
rotundala
Permian
Paleolimulus
avitus
Jurassic
Limulus
walchi
Recent
Limulus
polyphemus
or absence of genetic variation. Some living fossil species live in relatively isolated
habitats, with no apparent competitors, and if their habitats have been stable there
will have been no pressure for them to change. There is no evidence that living fossils
have peculiar genetic systems that might prevent evolutionary change, for instance by
constraining the degree of new genetic variation. The amount of genetic polymorphism
in modern Limulus polyphemus it is not noticeably low.
But the point of the example here is methodological, not biological. It is to show how
evolutionary rates can be studied quantitatively in characters whose evolutionary
changes are not simply metrical. The characters can be divided into discrete states; the
states assigned arbitrary scores; and the changes in those scores measured through
time.
The quantification is mainly useful for purposes of illustration. Figure 21.9 neatly
shows how rates of evolution have varied through time in a way that a table of raw
character data could not. But the division of characters into states, and the assignment
of scores to states, is arbitrary. The five states for supraorbital canal bone fusion could
just as well have been scored 40, 16, 15, 14, 0 or 2, 8, 17, 39, 40 as 4, 3, 2, 1, 0. It would
therefore be meaningless to compare exact numerical rates of change between characters,
or between taxa. The scores are incommensurable. The approximate shape of a
graph like Figure 21.10 could be compared with another such graph for another group;
but there would be no point in asking why one group changed at, say, 2.1 units per million
years and another at 1.3 units per million years. The scores are not intended for
that kind of analysis. But as an illustration of how rates of change in lungfish have risen
and fallen and declined to a virtual standstill, Westoll’s analysis is a classic.
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